Chapter 3: CELLS AND HOW THEY WORK

3 CellS and how they work Chapter Outline WHAT IS A CELL? All cells are alike in three ways There are two basic kinds of cells Most cells have a large surface area compared to their volume Membranes enclose cells and organelles ORGANELLES of a eukaryotic CELL How do we see cells? the plasma membrane: a DOUBLE LAYER OF LIPIDS The plasma membrane is a mix of lipids and proteins Proteins carry out most of the functions of cell membranes The plasma membrane is “selective” A WATEry disaster for cells The Nucleus A nuclear envelope encloses the nucleus The nucleolus is where cells make the parts of ribosomes DNA is organized in chromosomes Events that begin in the nucleus continue in the cell cytoplasm the endomembrane System ER is a protein and lipid assembly line Golgi bodies “finish, pack, and ship” Vesicles have a range of roles in cells MitochondriA: the cell’s energy factories Mitochondria make ATP ATP forms in an inner compartment of the mitochondrion THE CELL’S SKELETON HOW diffusion and osmosis MOVE SUBSTANCES ACROSS MEMBRANES In diffusion, a dissolved molecule or ion moves down a concentration gradient Each type of solute follows its own gradient Water crosses membranes by osmosis other ways substances cross cell membranes Many solutes cross membranes through the inside of transporter proteins Vesicles transport large solutes When mitochondria fail metabolism: doing cellular work ATP is the cell’s energy currency There are two main types of metabolic pathways Enzymes are essential in metabolism The body controls the activity of enzymes how cells make atp Cellular respiration makes ATP Step 1: Glycolysis breaks glucose down to pyruvate Step 2: The Krebs cycle produces energy-rich transport molecules Step 3: Electron transport produces many ATP molecules summary of CELLULAR respiration Other Energy Sources no thanks to arsenic Summary Review questions self-quiz critical thinking explore on your own YOUR FUTURE Objectives Understand the basic parts of eukaryotic cells. Describe the structure of the plasma membrane and its role in regulating the movement of material into and out of the cell. Explain the fluid mosaic model of the plasma membrane. Know the forces that cause water and solutes to move across membranes passively and by active transport. Describe the various functions of membrane proteins. Understand how material can be imported into or exported from a cell by being wrapped in membranes. Explain the cell theory. Compare light, fluorescence, transmission, and scanning electron microscopes. Distinguish between prokaryotes and eukaryotes. Explain the theory of endosymbiosis. Describe the nucleus of eukaryotes with respect to structure and function. Understand the genetic material in the form of DNA that resides within the nucleus of eukaryotes. Compare the following components of the endomembrane system: rough endoplasmic reticulum, smooth endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes, and vacuoles. Describe the structure and function of mitochondria and chloroplasts. Describe the cytoskeleton of eukaryotes and distinguish it from the endomembrane system. Define a metabolic pathway and the types of substances that participate in it. Characterize an enzyme and what type of cofactors may be needed for its functioning. Describe how a molecule can pass a cell membrane. Define ATP and describe the pathways for its formation within the cell. Describe the process of cellular respiration with special reference to the quantity of ATP produced. Key Terms cell theory plasma membrane DNA cytoplasm prokaryotic cell eukaryotic cell organelles surface-to-volume ratio lipid bilayer microscopy micrograph selective permeability nucleus nuclear envelope nucleolus chromatin chromosome endomembrane system endoplasmic reticulum ribosome Golgi body vesicle lysosome peroxisomes mitochondrion, -dria cytoskeleton microtubules microfilaments intermediate filaments flagella, flagellum cilia, cilium centrioles concentration gradient diffusion passive transport osmosis isotonic hypotonic hypertonic facilitated diffusion active transport endocytosis phagocytosis exocytosis metabolism ATP/ADP cycle anabolism catabolism enzymes substrates active site cellular respiration glycolysis phosphorylation Krebs cycle electron transport system Lecture Outline The liver can detoxify alcohol, but this puts a metabolic burden on the organ. Over time, this can cause severe and extensive damage to the liver. What Is a Cell? The cell theory has three generalizations. All organisms are composed of one or more cells. The cell is the smallest unit having the properties of life. All cells come from pre-existing cells. All cells are alike in three ways. A plasma membrane separates each cell from the environment, but also allows the flow of molecules across the membrane. DNA carries the hereditary instructions. The cytoplasm containing a semifluid matrix (cytosol) and organelles is located between the plasma membrane and the region of DNA. There are two basic kinds of cells. Prokaryotic cells (bacteria) do not have a separation of the DNA from the remainder of the cell parts. Eukaryotic cells have a definite nucleus and membrane-bound organelles. Most cells have a large surface area compared to their volume. Most cells are so small they can only be seen by using light and electron microscopes. Cells are necessarily small so that the surface-to-volume ratio remains low; this means the interior will not be so extensive that it cannot exchange materials efficiently through the plasma membrane. Membranes enclose cells and organelles. A large portion of the cell membrane is composed of phospholipids, each of which possesses a hydrophilic head and two hydrophobic tails. If phospholipid molecules are surrounded by water, their hydrophobic fatty acid tails cluster and a lipid bilayer results; hydrophilic heads are at the outer faces of a two-layer sheet with the hydrophobic tails shielded inside. Organelles of a Eukaryotic Cell All eukaryotic cells contain organelles. Organelles form compartmentalized portions of the cytoplasm. Organelles separate reactions with respect to time (allowing proper sequencing) and space (allowing incompatible reactions to occur in close proximity). A diagram of a typical animal cell is presented in Figure 3.5, and brief descriptions of the various organelles can be found in Table 3.2. How Do We See Cells? Microscopy allows us to see cells and their pieces. Many types of microscopes exist, which can produce many types of pictures (micrographs). Light microscopes use light to see samples; specimens usually must be thin and colored with dyes to be seen. Electron microscopes use beams of electrons rather than light to see details; transmission and scanning electron microscopy can magnify (enlarge) specimens far beyond the limits of the light microscope. The Plasma Membrane: A Double Layer of Lipids The plasma membrane is a mix of lipids and proteins. Bilayers of phospholipids, interspersed with glycolipids and cholesterol, are the structural foundation of cell membranes. Within a bilayer, phospholipids show quite a bit of movement; they diffuse sideways, spin, and flex their tails to prevent close packing and promote fluidity, which also results from short-tailed lipids and unsaturated tails (kinks at double bonds). Proteins carry out most of the functions of cell membranes. Membrane proteins (most are glycoproteins) serve as enzymes, transport proteins, receptor proteins, and recognition proteins. The plasma membrane is “selective.” The plasma membrane can control which molecules can pass. Most molecules move through membrane proteins; some small, non-polar molecules can move through the lipids. A Watery Disaster for Cells Cholera is caused by the bacteria Vibrio cholera. The bacteria release a toxin that causes cells to lose water. The resulting diarrhea causes a potentially fatal dehydration. Treatment involves antibiotics and rehydration therapy. The Nucleus The nucleus encloses DNA, the building code for cellular proteins. Its membrane isolates DNA from the sites (ribosomes in the cytoplasm) where proteins will be assembled. The nuclear membrane helps regulate the exchange of signals between the nucleus and the cytoplasm. A nuclear envelope encloses the nucleus. The nuclear envelope consists of two lipid bilayers with pores. The envelope membranes are continuous with the endoplasmic reticulum (ER). The nucleolus is where cells make the parts of ribosomes. The nucleolus appears as a dense mass inside the nucleus. In this region, subunits of ribosomes are prefabricated before shipment out of the nucleus. DNA is organized in chromosomes. Chromatin describes the cell’s collection of DNA plus the proteins associated with it. Each chromosome is one DNA molecule and its associated proteins. Events that begin in the nucleus continue in the cell cytoplasm. Outside the nucleus, new polypeptide chains for proteins are assembled on ribosomes. Some proteins are stockpiled; others enter the endomembrane system. The Endomembrane System ER is a protein and lipid assembly line. The endoplasmic reticulum is a collection of interconnected tubes and flattened sacs, continuous with the nuclear membrane. Rough ER consists of stacked, flattened sacs with many ribosomes attached; oligosaccharide groups are attached to polypeptides as they pass through on their way to other organelles, membranes, or to be secreted from the cell. Smooth ER has no ribosomes; it is the area from which vesicles carrying proteins and lipids are budded; it also inactivates harmful chemicals and aids in muscle contraction. Golgi bodies “finish, pack, and ship.” In the Golgi body, proteins and lipids undergo final processing, sorting, and packaging. The Golgi bodies resemble stacks of flattened sacs whose edges break away as vesicles. Vesicles have a range of roles in cells. Lysosomes are vesicles that bud from Golgi bodies; they carry powerful enzymes that can digest the contents of other vesicles, worn-out cell parts, or bacteria and foreign particles. Peroxisomes are membrane-bound sacs of enzymes that break down fatty acids and amino acids. Mitochondria: The Cell’s Energy Factories Mitochondria make ATP. Mitochondria are the primary organelles for transferring the energy in carbohydrates to ATP; they are found only in eukaryotic cells. Oxygen is required for the release of this energy. ATP forms in an inner compartment of the mitochondrion. Each mitochondrion has compartments formed by inner folded membranes (cristae) surrounded by a smooth outer membrane. Mitochondria have their own DNA and some ribosomes, which leads scientists to believe they may have evolved from ancient bacteria. The Cell’s Skeleton The cytoskeleton is an interconnected system of bundled fibers, slender threads, and lattices extending from the nucleus to the plasma membrane in the cytosol. The main components are microtubules, microfilaments, and intermediate filaments—all assembled from protein subunits. The skeleton helps organize and reinforce the cell and serves in some cell functions. Movement is one function of the cytoskeleton. Flagella and cilia are both microtubular extensions of the plasma membrane, displaying a 9 + 2 cross-sectional array, and are useful in propulsion. Flagella are quite long, whiplike, and found on animal sperm cells. Cilia are shorter, more numerous, and may function as “sweeps” to clear, as an example, the respiratory tract of dust or other materials. The microtubules of flagella and cilia arise from centrioles, which play a role in cell division. How Diffusion and Osmosis Move Substances across Membranes The plasma membrane is “selective.” Lipid-soluble molecules and small, electrically neutral molecules (for example, oxygen, carbon dioxide, and ethanol) cross easily through the lipid bilayer. Larger molecules (such as glucose) and charged ions (such as Na+, Ca+, HCO3¯) must be moved by membrane transport proteins. Because some molecules pass through on their own, and others must be transported, the plasma membrane is said to have the property of selective permeability. In diffusion, a dissolved molecule or ion moves down a concentration gradient. A concentration gradient is established when there is a difference in the number of molecules or ions of a given substance between two adjacent regions. Molecules constantly collide and tend to move from areas of high concentration to areas of low concentration. The net movement of like molecules down a concentration gradient (high to low) is called diffusion; when this occurs across a plasma membrane, it is called passive transport. Each type of solute follows its own gradient. Molecules move faster when gradients are steep, and different solutes move independently according to their respective gradients. Electric gradients (gradients of charged molecules) are important to nerve function. Water crosses membranes by osmosis. Osmosis is the passive diffusion of water across a differentially permeable membrane in response to solute concentration gradients. Osmotic movements are affected by the relative concentrations of solutes in the fluids inside and outside the cell (tonicity). An isotonic fluid has the same concentration of solutes as the fluid in the cell; immersion in it causes no net movement of water. A hypotonic fluid has a lower concentration of solutes than does the fluid in the cell; cells immersed in it may swell as water moves into the cell down its gradient. A hypertonic fluid has a greater concentration of solutes than does the fluid in the cell; cells in it may shrivel as water moves out of the cell, again down its gradient. Other Ways Substances Cross Cell Membranes Many solutes cross membranes through the inside of transporter proteins. In facilitated diffusion, solutes pass through channel proteins in accordance with the concentration gradient; this process requires no input of energy. Channel proteins are open to both sides of the membrane and undergo changes in shape during the movement of solutes. The transport proteins are selective for what they allow through the membrane. In active transport, solutes move against their concentration gradients with the assistance of transport proteins that change their shape with the energy supplied by ATP. Vesicles transport large solutes. Endocytosis encloses particles in small portions of plasma membrane to form vesicles that then move into the cytoplasm; if this process brings organic material into the cell, it is called phagocytosis. Exocytosis moves substances from the cytoplasm to the plasma membrane during secretion, moving materials out of the cell. When Mitochondria Fail Luft’s syndrome is a disease in which the mitochondria produce too little ATP. The resulting lack of cellular energy damages body parts with high energy needs. Metabolism: Doing Cellular Work ATP is the cell’s energy currency. Metabolism refers to all of the chemical reactions that occur in cells; ATP links the whole of these reactions together. ATP is composed of adenine, ribose, and three phosphate groups. ATP transfers energy in many different chemical reactions; almost all metabolic pathways directly or indirectly run on energy supplied by ATP. ATP can donate a phosphate group (phosphorylation) to another molecule, which then becomes primed and energized for specific reactions. The ATP/ADP cycle is a method for renewing the supply of ATP that is constantly being used up in the cell; it couples inorganic phosphate to ADP to form energized ATP. There are two main types of metabolic pathways. Metabolic pathways form series of interconnected reactions that regulate the concentration of substances within cells. In anabolism, small molecules are assembled into large molecules—for example, simple sugars are assembled into complex carbohydrates. In catabolism, large molecules such as carbohydrates, lipids, and proteins are broken down to form products of lower energy, releasing energy for cellular work. Pathways exist as enzyme-mediated linear or circular sequences of reactions involving the following: Reactants are the substances that enter a reaction. Intermediates are substances that form between the start and conclusion of a metabolic pathway. End products are the substances present at the conclusion of the pathway. Enzymes are essential in metabolism. Enzymes are proteins that serve as catalysts; they speed up reactions. Enzymes have several features in common: Enzymes do not make anything happen that could not happen on its own; they just make it happen faster. Enzymes can be reused. Enzymes act on specific substrates, molecules that are recognized and bound at the enzyme’s active site. Because enzymes operate best within defined temperature ranges, high temperatures decrease reaction rate by disrupting the bonds that maintain a three-dimensional shape (denaturation occurs). Most enzymes function best at a pH near 7; higher or lower values disrupt enzyme shape and halt function. Coenzymes are large organic molecules such as NAD+ and FAD (both derived from vitamins), which transfer protons and electrons from one substrate to another to assist with many chemical reactions. The body controls the activity of enzymes. The speed of enzyme function can be altered. Enzyme synthesis can be accelerated or reduced. How Cells Make ATP Cellular respiration makes ATP. Electrons acquired by the breakdown of carbohydrates, lipids, and proteins are used to form ATP. Overall, the formation of ATP occurs by cellular respiration; in humans this is an aerobic process, meaning it requires oxygen. Step 1: Glycolysis breaks glucose down to pyruvate. Glycolysis reactions occur in the cytoplasm and result in the breakdown of glucose to pyruvate, generating small amounts of ATP. Glucose is first phosphorylated in energy-requiring steps, and then split to form two molecules of PGAL. Four ATP are produced by phosphorylation in subsequent reactions; but because two ATP were used previously, there is a net gain of only two ATP by the end of glycolysis. Glycolysis does not use oxygen. Step 2: The Krebs cycle produces energy-rich transport molecules. Pyruvate (produced in the cytoplasm) enters the mitochondria for the oxygen, requiring steps of cellular respiration. The pyruvate is converted to acetyl-CoA, which enters the Krebs cycle to eventually be converted to CO2. Reactions within the mitochondria and the Krebs cycle serve three important functions: Two molecules of ATP are produced by substrate-level phosphorylation. Intermediate compounds are regenerated to keep the Krebs cycle going. H+ and e¯ are transferred to NAD+ and FAD, generating NADH and FADH2. Step 3: Electron transport produces many ATP molecules. The final stage of cellular respiration occurs in the electron transport systems embedded in the inner membranes (cristae) of the mitochondrion. NADH and FADH2 from previous reactions give up their electrons to transport (enzyme) systems embedded in the mitochondrial inner membrane. Electrons flow through the system eventually to oxygen, forming water; as they flow, H+ are pumped into the outer compartment of the mitochondrion to create a proton gradient. H+ ions move down their gradient, through a channel protein called ATP synthase, in the process driving the synthesis of ATP. Summary of Cellular Respiration Figure 3.26 shows a summary of the steps of aerobic cellular respiration. In total, glycolysis, the Krebs cycle, and the electron transport system can yield a maximum of 36 ATP per glucose molecule. Other Energy Sources Glucose from carbohydrates is the body’s main energy source. Excess carbohydrate intake is stored as glycogen in liver and muscle for future use. Free glucose is used until it runs low; then glycogen reserves are tapped. Under some conditions a process called lactate fermentation can be used to produce ATP; here, pyruvate is converted directly to lactic acid with production of quick, but limited, energy. Fats and proteins also provide energy. Lipids are used when carbohydrate supplies run low. Excess fats are stored away in cells of adipose tissue. Fats are digested into glycerol, which enters glycolysis, and fatty acids, which enter the Krebs cycle. Because fatty acids have many more carbon and hydrogen atoms, they are degraded more slowly and yield greater amounts of ATP. Proteins are used as the last resort for supplying energy to the body. Amino acids are released by enzymatic digestion of proteins; protein is never stored by the body. After the amino group is removed, the amino acid remnant is fed into the Krebs cycle to produce energy (ATP), or is used to make fats and carbohydrates. Suggestions for Presenting the Material This chapter builds on the chemical foundations laid in Chapter 2, so encouraging students to refer often to Chapter 2 will help them understand the current material. For many readers, Chapter 3 represents the real entry into the realm of biology. Indeed, a discussion of the cell and its functions is fundamental to all future lectures. If the discussion of phospholipid function from Chapter 2 was deferred to the present chapter, now it should be resumed. Be careful to show that all of the phospholipid diagrams in the present chapter are characterizations of Figure 2.22. Do not assume students know, or remember, the definitions of “hydrophilic” and “hydrophobic.” Using the transparency or a PowerPoint slide of Figure 3.7 (view of plasma membrane); carefully distinguish between which portion of the membrane is the “fluid” (that is, lipid bilayer) and which is the “mosaic” (that is, protein). Use demonstrations of mosaic artwork (see the “Classroom and Laboratory Enrichment” section below) if it can be located. The various methods by which molecules move, either through space or through membranes, can be confusing to students because of the subtle differences that distinguish each method. Perhaps begin with general, nonmembrane-associated phenomena such as diffusion. Then proceed to membrane-associated mechanisms such as osmosis, facilitated diffusion, active transport, and vesicle formation. Note that each of the transport phenomena topics is accompanied by an illustration, which should be used to reduce the abstract quality of the mechanism. A more complete historical sketch of the development of the cell theory and its three elements should be included to draw students in and demonstrate the scientific method in process: 1) all living things are composed of cells, 2) cells are the functional (and dysfunctional) units of life, and 3) all cells come from cells. This may also be an excellent time to review the use of the word “theory” as explained in Chapter 1. A clear distinction between prokaryotic and eukaryotic cells should be made (see the “Classroom and Laboratory Enrichment” section below for visual aid suggestions). When describing cell organelles, use the slide of Figure 1.5 (the “levels of organization”) to remind students of the progress they are making. Although the descriptions and diagrams of the cell organelles occupy only a small amount of textbook space, it is best to proceed carefully and deliberately. There is a wide array of unfamiliar terms here that students need to become comfortable with before moving on from this chapter. When describing each cell structure, a visual representation of some type should be constantly in view of the student. Each time a new cell part is introduced, Figure 3.5 should be shown for reference purposes. Stress the fact that some cell parts are so complex in function that greater detail will follow in future lectures. If specific examples are preferred, choose a specific metabolic pathway (many can be found in the textbook), draw it on an overhead transparency, and use it to explain the various terms found in the text. After presentation of the various capabilities of enzymes, students may think of them as “miracle workers.” Remind the students that these are nonliving molecules—albeit, amazing ones. Also emphasize the limitations and vulnerability of enzymes, including causes and effects of denaturation. Instruct students to bring their texts to class and refer frequently to the excellent figures in it. The critical role of ATP must be emphasized. Distinguish clearly between the transfer of energy from carbohydrates to ATP and the synthesis of the ATP molecule. The steps of cellular (aerobic) respiration are most easily and logically presented to students by using Figure 3.26. Emphasize the processes taking place rather than requiring students to memorize the various reactions. As a review exercise, ask students to fill the quantities for the energy yields in the figure found in the “Summary” on page 65 of the text. Although the focus in the latter part of this chapter is the generation of ATP, explain to students that another major function of respiration is the production of intermediates for biosynthetic reactions. Classroom and Laboratory Enrichment Unless a student has an interest in art, he/she may not know what a “mosaic” is. Bring an example of such a piece of art, or at least a photo, it will aid the description of the fluid mosaic model. Demonstrate the structure of the plasma membrane with a three-dimensional model, overhead transparencies, or PowerPoint slides. Use electron micrographs (in the form of overhead transparencies or CD images) to add to your description of plasma membrane structure. Use sketches or models drawn to scale to demonstrate the size difference between prokaryotic and eukaryotic cells. Show an overhead transparency of a diagram or an electron micrograph of any cell. Ask if the cell is prokaryotic or eukaryotic. Is it an animal cell? A plant cell? Some other type of cell? What characteristics do the students use to determine the cell type? Construct a table (overhead or handout) listing side-by-side comparisons of prokaryotic and eukaryotic cells. Most departments possess some type of cell model. These are especially helpful in perception of the 3D aspects of cell structure. They can also be useful in oral quizzing. Ask students to match “organelle” with “cellular task” at the board or on an overhead. If presenting the historical sketch of the cell theory, include slides of the researchers being discussed. These photos can usually be found in a variety of introductory biology texts or special texts on the history of biology. Try bringing some electron micrographs, to class to pass around, or find images online to project during the lecture. Many universities, biological societies, and journals post micrographs on their websites (electron micrographs, fluorescent micrographs, etc.). Use these images to show students the “real” cell as well as the beauty of biological structures. Show a video that reviews cell structures and functions, especially if cells from different tissues or organisms are compared. Use models or overhead transparencies to show how channel proteins and transport proteins function during passive transport and active transport, respectively. Open a bottle of perfume and place it on the desk early in the lecture period. Discuss principles involved later in the lecture. Alternatively, bring a can of room deodorizer spray to class to demonstrate diffusion (in this case, a liquid in a gas) as a general example of how concentration gradients operate. To show how molecules move from points of greater concentration to regions of lower concentration, pour red ink or dye along one side of a container of water. The liquid will diffuse through a liquid. Discuss the terms “isotonic,” “hypotonic,” and “hypertonic” by showing students three sketches of semipermeable bags in beakers of distilled water (these can be drawn ahead of time on an overhead transparency if you wish). Vary the concentrations of the sugar solutions shown in the bags. Ask students what the direction of water movement would be in each case. Then ask in what direction the sugar molecules will move. Many students will believe that the sugar molecules will move across the membrane, even though they previously learned these sugar molecules are too large to cross the plasma membrane readily. In the classroom, demonstrate the loss of turgor pressure by flooding a small potted tomato plant with salt water at the beginning of the class period. If allowed by the institution, and while modeling all proper safety precautions and personal protective equipment, incorporate the following demonstrations into lecture to illustrate some of concepts being discussed: The diffusion of a gas through other gases (air) can be demonstrated easily as follows: Wet a circle of filter paper with phenolphthalein and insert it into the bottom of a large test tube. Next, invert the test tube over an open bottle of ammonium hydroxide. Ask students to explain the rather rapid color change of the filter paper to red. Set up a control with filter paper soaked in water. Demonstrate the diffusion of a gas in a gas using a glass tube at least one-half inch wide and 18 inches in length. Plug one end of the tube with a cotton wad saturated with HCl (hydrochloric acid); plug the opposite end with a cotton wad saturated with NH4OH (ammonium hydroxide). Label each end appropriately. Hydrochloric acid and ammonium hydroxide react together to produce ammonium chloride and water. Students will be able to see a ring of ammonium chloride that has formed closest to the HCl end of the tube; the ammonium ion (NH4+) has a smaller molecular weight than the chloride ion (Cl¯) and thus diffuses faster, meeting the Cl¯ ion about two-thirds of the way down the tube. Demonstrate the diffusion of a liquid in a solid. Prepare three Petri dishes, each filled with a layer of plain agar. Use a cork borer to carefully make three equidistant holes in the agar. Turn the dish over and number each hole. Put three or four drops of the following solutions in each of the appropriately numbered holes: (1) 0.02M potassium dichromate, (2) 0.02M potassium permanganate, and (3) 0.02M methylene blue. View results after at least one hour. Because the solutions have identical molarities, the rate of diffusion depends on the molecular weight of each compound. Methylene blue (MW = 373) diffuses at the slowest rate, while potassium permanganate diffuses fastest (MW = 158). Potassium dichromate (MW = 294) diffuses at an intermediate rate. At least one day before lecture, dissolve gelatin in water and pour into a screw-top test tube; leave a small space between the top of the gelatin and the top edge of the test tube. Cool the tube in the refrigerator until the gelatin solidifies. At the beginning of the lecture, pour in a small amount of a bright-colored dye on top of the gelatin; replace and tighten the screw top. Allow the dye to diffuse through the gelatin until it reaches the bottom of the glass thread area of the test tube. At this time, turn the test tube to a horizontal position and place it so that students can observe progress of the dye through the gelatin. If one wishes to time the progress of the dye, begin timing when the tube is turned to the horizontal position. To demonstrate that some molecules will pass through membranes and some will not, prepare two test tubes as follows: In one tube pour dilute Lugol’s iodine solution until it is about nine-tenths full; in another tube pour 1% starch paste until it is about nine-tenths full. Cover the mouth of each test tube with a wet goldbeater’s membrane; then secure it tightly by tying with thread or a tight rubber band. Invert the starch paste test tube into a beaker about one-half full of dilute Lugol’s solution; invert the test tube containing dilute Lugol’s solution into a beaker containing a 1% starch paste. Ask students why the well-known blue-black color appears in the starch solution and not in the Lugol’s solution. Students may decide that the starch requires digestion to a more soluble form before it can pass through the membrane. Set up two or more osmometer tubes (glass or plastic thistle tubes covered with a selectively permeable membrane) at the front of the room. Compare rates of osmosis by filling each tube with a colored sugar (you may use corn syrup) or salt solution (vary the concentrations) and placing the base of the tube into a beaker of distilled water. Living cells can be used to demonstrate the osmotic water passage through semipermeable membranes. Use an apple corer to remove a center cylinder of a raw white potato; leave about one-half inch of potato tissue at the bottom. Carefully pour a concentrated sucrose solution into the core; seal the opening with a one-hole rubber stopper through which a piece of glass tubing has been inserted. Pour melted paraffin around the stopper as a seal to avoid leakage. Place the potato in a beaker of water; use a clamp on a ringstand to support the glass tube. In the lab, students can view plasmolysis under the microscope. Obtain a small Elodea leaf and place it in a small drop of distilled water on a microscope slide. Cover it with a cover slip. Now prepare a second slide, only this time mounting an Elodea leaf in a drop of 10% NaCl solution. Compare the cells of the second slide to those of the first slide. The action of an enzyme (salivary amylase) can be easily demonstrated by the following procedure: Prepare a 6% starch solution in water and confirm its identity by a spot plate test with iodine solution (produces blue-black color). Collect saliva from a volunteer by having the person chew a small piece of Parafilm and expectorate into a test tube. Place diluted saliva and the starch solution in a test tube and mix. At suitable intervals, remove samples of the digestion mixture and test with iodine on the spot plate (lack of dark color indicates conversion of starch to maltose). Variations can include: heating the saliva to destroy the enzyme, adding acid or alkali, and adding cyanide. Demonstrate the two models of enzyme-substrate interactions in the following ways: Rigid “lock and key” model: Use preschool-size jigsaw puzzle pieces or giant Lego® blocks. Induced-fit model: Use a flexible fabric or latex glove to show how the insertion of a hand (substrate) induces change in the shape of the glove (active site). Show a video depicting the role and function of enzymes. The effect of ATP on a reaction can be demonstrated by use of bioluminescence kits available from biological supply houses. The relationship among ATP, ADP, energy, enzymes, and phosphorylation may be illustrated by the use of a toy dart gun with rubber suction cup-tipped darts. It is helpful to have acetate transparencies of ADP and ATP structures that can be projected on a screen as the following demonstration is performed: Tell the students that the unloaded dart gun represents ADP and the dart represents inorganic phosphate (Pi). Show the structure of ADP on the screen. As you insert the dart into the gun, emphasize the need for the expenditure of energy to do this. Tell the students that the addition of Pi to ADP is, therefore, an endergonic reaction; it is also called a phosphorylation reaction. At this time show the structure of ATP on the screen. Also point out that the spring inside the dart gun is under much tension, and as such has a great deal of potential energy. The same can be said for the Pi group that has been added to ADP. Next, demonstrate the hydrolysis of ATP. The trigger finger represents the necessary enzyme. Aim the gun at some vertical smooth surface (window or aquarium works well) and depress the trigger. The dart should adhere to the surface (a substrate molecule being energized by phosphorylation). The reaction is thus exergonic and some of the energy has been transferred to the substrate molecule. Show a video about energy transformations in cells. Show a video on cellular respiration. If there is a brewery or winery nearby, arrange for a field trip. Brewmasters and winemakers generally are happy to conduct a tour through the facilities and explain the anaerobic processes involved. Ask an exercise physiologist to talk to the class about the effect(s) of exercise on body metabolic rate. Relate this talk to concepts of proper nutrition and the problem of obesity in America. Classroom Discussion Ideas Explain the connection between repeated exposure to toxins and damage to the body. Ask the class to come up with means by which we damage our bodies. Extend the debate about organ transplants to obtaining medical assistance at all. People claim that drinking more expensive alcohols are better for you. Is there truth to this claim? Why do we use the fluid mosaic model to describe the plasma membrane? Why is the structure of the plasma membrane basically the same among all known organisms? Ask students to think about the common evolutionary origins of all life. What would happen to freshwater unicellular organisms if they were suddenly released in a saltwater environment? What are some organelles that contain internal compartmentalizations? How do internal compartments assist in the functioning of the organelle? Why are organelles necessary in larger eukaryotic cells? Distinguish between diffusion and osmosis. Why do unicellular protists (such as Paramecium) not burst even though their cell interiors are hypertonic relative to their freshwater environments? What is physiological saline solution? (Hint: It is used to dilute samples of red blood cells in the laboratory.) What would be the result on blood cells of a substitution of pure water for physiological saline in an IV bottle? How do exocytosis and endocytosis differ from passive transport and active transport? What is “fertilizer burn”? What can be done to correct it? Someone has said that a diagram of the plasma membrane resembles a scene from the North Atlantic. What they are referring to are “icebergs” representing _______________ (molecules) floating in a “sea” of ________________ (molecules). Based on your knowledge of membranes and solubility, which insecticide preparation would you expect to kill insects faster: one that is water formulated or petroleum-solvent formulated? In some cases, a 5% glucose solution is given intravenously to persons after surgery. If you were the doctor on such a case, would you order the glucose solution to be isotonic, hypertonic, or hypotonic to blood? Explain your decision. Discuss the precise meanings of the prefixes hyper- and hypo-. They are often confused by students, and mental errors are compounded. If they do not understand that these terms refer to solute concentration (not water), they will have difficulty with the concept. Plant cells have a rather rigid wall enclosing their plasma membranes; animal cells do not. Ask students to think about a comparison of consequences when plant and animal cells are placed in isotonic, hypertonic, and hypotonic solutions. List the tasks that a cell must perform. What is the largest example of a single cell that you can think of? Why are there no unicellular creatures one foot in diameter? Why must bacteria have ribosomes when they lack other organelles? What is the difference between scanning electron microscopy and transmission electron microscopy? Why do we need both types of microscopy in addition to other techniques to get a true picture of what a cell looks like? Where in your body would you find cells with high concentrations of mitochondria? Why do you think most plant cells have a central vacuole while animal cells lack this organelle? What is the significance of the word “theory” in reference to the basic properties of the cell? Does the cytoplasm have any functions of its own, or is it just a “filler” matrix in which other organelles float? Why is the term “nucleus” used to describe the center of an atom and the organelle at the center of the cell, when these are such different entities? In measurement of length, what are the largest cells (when mature) in the human body? What fundamental property of all cells is denied to these? What feature makes a eukaryotic cell a “true” cell? What organelle could be compared to the control center of an assembly line in a factory? Describe the interrelationship(s) of the individual members comprising the endomembrane system (see Figure 3.12). Pose several “what if” questions concerning enzyme action, such as: What if the body temperature of humans mutated to 105°F? Or body pH climbed to 9.5? Preview the “inborn errors of metabolism” wherein a faulty enzyme blocks or diverts a metabolic pathway, as in phenylketonuria (PKU). Compare the energy “hill” that an enzyme must overcome to a ski slope. Does an enzyme act more like a chair lift or a bulldozer? Discuss what would happen to life on Earth if the flow of sunlight energy stopped. Name some of the different forms of energy. Are they interconvertible? Give some examples of interconversions. For example, trace the energy interconversions involved in cooking your breakfast on an electric stove. Begin with the dam over a hydroelectric plant. What is “metabolic water”? Why is fermentation necessary under conditions where there is no oxygen? That is, why does the cell convert pyruvate to some fermentation product when it does not result in any additional ATP production? Term Paper Topics, Library Activities, and Special Projects What happens in arid climates when salts accumulate in cropland soils? How do cells “recognize” other cells of the same type during tissue formation? How are saltwater fish species able to cope with their extremely salty surroundings? From your library shelves, select a biology text written 20 years ago. Turn to the discussion of the ultrastructure of the plasma membrane. How does that model differ from your text’s fluid mosaic one? Go back even further; search the library shelves for biology texts of 30, 40, and 50 years ago. Find the diagrams of cell structure. Prepare a sequential composite of these, and compare each to the others and to your present text. What instrument made the difference in the drawings? Look up the composition of “physiological saline.” Are there different varieties of this preparation for different animal species? The diffusion and transport phenomena discussed in the chapter are based on a property called Brownian movement, which is demonstrated by all molecules, whether alive or not. What is the physical manifestation of this property, and what is the derivation of its name? Who first coined the term “organelle”? When did biologists discover that eukaryotic cells contained organelles? Learn more about plant tissue culture. What mechanisms govern cell differentiation in vitro? Discuss the development of electron microscopy. What are some of the advances in cell biology that electron microscopy has made possible? How are biological specimens prepared for examination with an electron microscope? How do antibiotics such as penicillin stop bacterial growth? Design a hypothetical cell that would function with maximum efficiency under conditions of extreme drought. Describe the function of liver cell smooth ER in the metabolism of drugs and alcohol. How might the liver cells of an alcoholic differ from those of a moderate imbiber or nondrinker? Prepare brief biographies of the researchers who are credited with early discoveries of cell structure and function. Using a special dictionary of Latin and Greek root words, search for the literal meanings for each of the cell parts listed in your textbook. (Be careful with Golgi—it is a man’s name!) Explain the defect in the metabolic pathway that results in the condition known as phenylketonuria (PKU). How can this condition be treated? Is it curable? Construct clay models depicting an enzyme, a substrate, and the enzyme-substrate complex, which together illustrate the concept of induced fit. Explain the role of the B vitamins in human metabolism. Discuss some of the ways coenzymes and inorganic cofactors participate in enzymatic reactions. Rotenone is a fish poison and insecticide. Its mode of action is listed on container labels as “respiratory poison.” Exactly where and how does it disrupt cellular respiration? Certain flour beetles and clothes moths can live in environments where exogenous water is virtually unobtainable, yet they thrive. What mechanisms do they use for the synthesis and retention of water? Use diagrams to show how radioactive carbon 14 in glucose fed to rats could end up in body fat and proteins. Because ATP is the direct source of energy for body cells, why not bypass the lengthy digestion and cellular metabolism processes necessary for carbohydrate breakdown and eat ATP directly? Investigate the “set-point theory” of metabolism; discuss how it relates to people who are trying to lose or gain weight. Hydrogen cyanide is the lethal gas used in gas chambers. How does it cause death? Videos, Animations, and Websites VIDEOS Harvard University A music-only video of an animated view of the inside of a cell. http://www.xvivo.net/the-inner-life-of-the-cell/ How Stuff Works This page contains a video that gives a good overview of cells. http://science.howstuffworks.com/environmental/life/cellular-microscopic/cell.htm ANIMATIONS University of Utah – Genetic Science Learning Center Interactive overview of the inside of a cell (either animal or plant). http://learn.genetics.utah.edu/content/begin/cells/insideacell/ Wisconsin Technical College System: WISC Online – Interactive Eukaryotic Cells An interactive animation of the organelles of an eukaryotic cell. http://www.wisc-online.com/Objects/ViewObject.aspx?ID=AP11403 Wisconsin Technical College System: WISC Online – Cell Membranes An interactive animated overview of plasma membranes. http://www.wisc-online.com/Objects/ViewObject.aspx?ID=ap1101 Wisconsin Technical College System: WISC Online – Interactive Prokaryotic Cells An interactive animated overview of a prokaryotic cell. http://www.wisc-online.com/Objects/ViewObject.aspx?ID=MBY901 WEBSITES University of Arizona – The Biology Project Resource on various aspects of cell biology. http://www.biology.arizona.edu/cell_bio/cell_bio.html Cells Alive! Resource on various aspects of cell biology. http://www.cellsalive.com/ University of Miami – Introductory Biology Course The history of cell theory. http://fig.cox.miami.edu/~cmallery/150/unity/cell.text.htm Earthlife An overview of prokaryotic cells. http://www.earthlife.net/prokaryotes/welcome.html Possible Responses to Review Questions The plasma membrane forms a semipermeable barrier between the inside of the cell and the external environment. It essentially “forms” the cell and controls what goes in and comes out of the cell. The cytoplasm or cytosol is the semi-liquid gel of materials (proteins, nutrients, wastes, etc.) that provide the cell with everything it needs to function. DNA contains the “instructions” for the cell, spelling out how the cell forms, what its purpose is, when it dies, and all other functions. Ribosomes take messages from DNA and turn them into proteins for the cell. Organelles take varying forms, but all are discrete, membrane-associated entities dedicated to specific purposes in the cell. Mitochondria, for example, produce energy for the cell. The cytoskeleton gives structure to the cell and allows for specific movement-related functions. The endomembrane system consists of the endoplasmic reticulum (ER, both rough and smooth), the Golgi body, and vesicles of varying types—from endocytic and exocytic vesicles to lysosomes and peroxisomes. Diffusion is the non-energy dependent movement of small molecules through the plasma membrane; osmosis is specifically the non-energy dependent movement of water across the membrane. Passive transport involves channel proteins that facilitate the movement of small molecules across the membrane, but without the use of energy. For diffusion, osmosis and passive transport, molecules are always moving down their concentration gradients. In active transport, larger molecules are moved with the contribution of energy through transport proteins; often, this movement is against a concentration gradient. Endocytosis is the process whereby vesicles form at the plasma membrane, “grabbing” substances and bringing them into the cell. Exocytosis is essentially the opposite; vesicles from the endomembrane system carry substances to the cell surface for release. An enzyme is a protein specifically designed to facilitate the occurrence of specific chemical reactions. Enzymes reduce the energy required for a reaction to occur, thus speeding it up. Enzymes are not used up in the process, so they can continue to function within metabolic pathways. These proteins, essentially, make metabolism more efficient and fast enough to support life. Glycolysis occurs in the cytoplasm, while the Krebs cycle and electron transport occur in the mitochondria of the eukaryotic cell. For the diagram shown, refer to Figure 3.26 on page 62 of the text for answers. Possible Responses to Critical Thinking Questions The key word in the question is surface. To observe the surface of any object you would use a scanning electron microscope because it bounces electrons off the surface and is the instrument of choice here. Both the compound light microscope and the transmission electron microscope require the transmission of light or electrons, respectively, through the specimen (which must therefore be sliced very thin). Sprinting in the 100-meter dash takes a very short amount of time, during which the oxygen and ATP needs are supplied mostly by the reserves already in the body. No doubt there will be increased breathing and heart activity immediately after completion of the sprint. Golf, of course, is a more leisurely game (especially if a golf cart is used) and consumes oxygen and ATP at a slower, measured rate. It would be an overstatement to say it is a “non-aerobic” exercise. Jogging, of course, is a prolonged exercise and demands greatly increased amounts of oxygen to generate the ATPs needed for the muscle activity. If arsenic were to substitute itself in place of the distal phosphorus in an ATP molecule it would deeply impair the ability of that molecule to store and transfer energy. Nucleic acids are not constructed to enter any of the energy-generating metabolic pathways, and for good reason—they are much too valuable as informational molecules to be expended for energy. Possible Responses to Explore on Your Own Questions The cracker, which is mostly carbohydrate, will begin to disintegrate in the mouth (sooner if the cracker is a “soft” cracker like a saltine; slower if it is a “hard” cracker); this disintegration is associated with the beginnings of digestion. One major constituent of saliva is salivary amylase, which breaks down sugars. The butter or margarine might seem like it dissolves, but really it is only becoming emulsified within the saliva; lipid digestion occurs later in the digestive process. Proteins, such as meat, won’t dissolve no matter how long you hold it in your mouth—proteins require much different conditions for digestion. Thus, salivary enzymes work mostly on carbohydrates. 50 Chapter Three Cells and How They Work 51 50 Chapter Three Cells and How They Work 51 50 Chapter Three Cells and How They Work 51 50 Chapter Three Cells and How They Work 51 50 Chapter Three Cells and How They Work 51